Try this:
1. Create 3 points
PT1(X=100 Y=0 Z=0)
PT2(X=0 Y=50 Z=0)
PT3(X=0 Y=0 Z=30)
(You can create parametrs to control these points)
2. Create a Sketch.1 on XY plane
Draw an ellipse an trim it to have only less than 1/4
Use PT1 and PT2 to constraint the end points
3. Create a Sketch.2 on YZ plane
Draw an ellipse an trim it to have only less than 1/4
Use PT2 and PT3 to constraint the end points
4. Create a Sketch.3 on XZ plane
Draw an ellipse an trim it to have only less than 1/4
Use PT1 and PT3 to constraint the end points
5. Create a Sweep surface
Profile type: Explicit
Profile: Sketch.2
Guide curve: Sketch.1
Guide curve 2: Sketch.3
6. Then Symmetry, Join,....
To create an Ellipsoid, a simple approach is to model it in the unique case where all its semi-axes (a,b,c) are equal, the Sphere, and then stretch it to the dimensions required.
So, construct a semi-circle, centred on the origin and lying on one of the principal planes.
Probably far easier to do this without using Sketcher, it is not needed.
Create the sphere as a surface of revolution, Insert=>Surfaces=>Revolve, and then stretch it along one of its principal axes using Insert=>Operations=>Affinity as required. All this is covered in the Help Documentation, if not familiar with this transformation.
The Ellipsoid is a quadric surface so the resultant element of an Affinity Transform on a sphere will be mathematically pure, no sag, distortion or discrepances of tangency and curvature at join boundaries, etc, that can creep in when using less appropriate methods. By doing this all that is in effect happening is that the variables (a,b,c) in the equation of an ellipsoid are being changed. After all, an ellipse is a stretched circle and a circle is an ellipse of identical semi major and minor axes.
